This invention relates to determining transmit signal powers of channels in a CDMA (Code Division Multiple Access) communications system, in particular so-called reverse link channels from a remote station to a base station (BS) in a cellular communications system. The remote station is typically a mobile station (MS) and is referred to herein as such, but it may alternatively have a fixed location.
CDMA communications systems currently being developed, such as those referred to as IS-2000 and wideband CDMA systems, are desired to be able to provide a plurality of data channels on the reverse link from an MS to a BS. For example, such reverse link channels may include a dedicated control channel, a fundamental channel, and a supplementary channel. Each of these channels is desired when active to provide an individual data rate and quality of service (QoS). The QoS generally corresponds to a maximum bit error rate (BER) or frame error rate (FER) for typically 20 ms frames of information on the reverse link. For example, in an IS-2000 system, the control channel C can serve for carrying reverse link control information with a data rate of 9.6 kbps and a QoS requirement corresponding to a maximum BER of 10xe2x88x923, the fundamental channel F can serve for carrying voice communications with a data rate up to 9.6 kbps (Rate Set 1) or 14.4 kbps (Rate Set 2) and a QoS requirement corresponding to a maximum FER of 10xe2x88x922, and the supplemental channel S can serve for carrying other data (for example a video or multi-media signal) with a data rate for example up to about 1 Mbps and a QoS requirement corresponding to a maximum BER of 10xe2x88x926.
To facilitate channel estimation and coherent detection of the reverse link signal at the BS, the MS also provides on the reverse link a pilot channel P for a pilot signal on its own or time division multiplexed with other data. In the MS, the reverse link channels are orthogonally spread for example using respective Walsh codes and are then combined; the resulting signal is spread using a PN (pseudo random number) sequence for transmission on the reverse link from the MS to the BS.
In active operation of the MS, the pilot channel P is typically continuously on, and the other channels may be selectively present. In particular, there may be three configurations in which respectively only the channel F, only the channels C and F, and all of the channels C, F, and S are active with the pilot channel P.
It can be appreciated that, with simultaneous communications from multiple MSs to a BS in a cell of the system, the reverse link signal of each individual MS must be received by the BS and distinguished from noise or interference (referred to below simply as noise, for convenience) which includes the reverse link signals of all other MSs. For maximum system capacity, it is therefore very important that each MS transmit its reverse link signal at an optimum power or level, i.e. at a power which is just sufficient to permit recovery of each of its constituent data channels C, F, and/or S at its respective data rate and QoS, without contributing unduly to noise for all other MS reverse link signals. For similar reasons, it is necessary for each MS to combine its constituent channels with relative levels or gains that are optimally balanced, so that each channel has a signal level that is just sufficient for the BS to recover the channel at the respective bit rate and QoS.
To this end, it has been proposed that the MS, when it is powered on, initially determine what power of the pilot channel is required for recovery of the signal of each of the other channels C, F, and/or S individually at its respective data rate and QoS, to select as the power of the pilot channel the greatest one of the pilot channel powers thus determined, and to determine the power of each of the other channels C, F, and/or S relative to the selected power of the pilot channel. This can be done conveniently using a power balance default table, stored in the MS, for the respective configurations, data rates, and QoS requirements. The powers of the channels can be adjusted in known manner during subsequent communications between the MS and the BS.
An object of this invention is to provide an improved method of determining transmit signal powers of channels on the reverse link of a CDMA communications system.
In accordance with one aspect of the invention there is provided a method of determining relative signal powers for transmission of channels on a reverse link from a remote station to a base station of a CDMA (code division multiple access) cellular communications system, comprising the steps of: for each of a plurality of data channels for transmission, each of the data channels providing a respective data rate and a respective maximum error rate, determining a respective energy to noise ratio for the channel; for each of the plurality of data channels, determining a respective energy to noise ratio of a signal of a pilot channel required for recovering a signal of the data channel; selecting a maximum one of the respective required energy to noise ratios as an energy to noise ratio for transmission of the signal of the pilot channel; and determining a relative signal power for transmission of each of the plurality of data channels from the energy to noise ratio determined for the respective data channel and the selected maximum energy to noise ratio for transmission of the pilot channel.
The method preferably comprises the steps of spreading signals of the plurality of data channels using respective orthogonal codes, and combining the signals of the data channels with the signal of the pilot channel with relative gains dependent upon the determined relative signal powers for transmission of the respective channels.
Preferably the plurality of data channels comprise at least a fundamental channel providing a relatively low data rate for a voice signal and a supplemental channel providing a relatively high data rate and a maximum error rate less than a maximum error rate of the fundamental channel. Typically in this case the maximum energy to noise ratio selected for transmission of the signal of the pilot channel is the energy to noise ratio required for recovering the signal of the supplemental channel, and the signal power determined for transmission of the fundamental channel is less than the signal power determined for transmission of the signal of the pilot channel.
Another aspect of the invention provides a method of determining gains of signals of each of a plurality of data channels relative to a signal of a pilot channel, said signals being spread using orthogonal codes, for combining said signals for transmission from a mobile station to a base station of a CDMA (code division multiple access) communications system with an initial power sufficient for recovery of the signals of the data channels at the base station each with a respective data rate and a respective maximum error rate, comprising the steps of: (a) for each of the data channels, determining a signal bit energy to noise ratio in accordance with the maximum error rate of the data channel; (b) for each of the data channels, determining a respective channel energy to noise ratio from the determined signal bit energy to noise ratio and a signal processing gain for the data channel; (c) for each of the data channels, determining a respective pilot channel energy to noise ratio sufficient for recovery of the signal of the data channel by the base station; (d) selecting a maximum one of the respective required pilot channel energy to noise ratios as an energy to noise ratio for transmission of the pilot channel; and (e) determining relative gains for combining the signals of the data channels with the signal of the pilot channel from the respective data channel energy to noise ratio determined in step (b) and the pilot channel energy to noise ratio selected in step (d).